The present invention pertains to communication systems and more particularly to adaptive rate voice communication systems.
Present day digital voice communication systems are subject to degraded speech quality for changing channel conditions because of large variations in digital performance parameters such as bit error rate or received bit energy to noise density ratio. Such systems are only usable over a slight range of these varying channel conditions.
In a fixed aggregate rate system the source and channel coding rates are allowed to vary, but the whole sum of the voice coding bit rate plus the channel coding bit rate is constant. Thus, if the aggregate rate is fixed, the sum of the rates of the source and channel coding is equal to the aggregate channel rate. The prior art discloses the use of a variable rate speech and a variable channel coding rate, but at a fixed aggregate channel rate, the sum of the speech (source) bit rate and the channel coding rate always being fixed. In addition, such prior art systems also use a fixed modulation method.
Channel conditions include such parameters as: speech quality, intelligibility, the signal-to-noise ratio (S/N), symbol error rate (SER) and bit error rate (BER) of the transmitted and received signal. In typical wireless communication systems, degradation of the communication channel conditions may inhibit quality communication or may prevent communication at all.
Link margins of such communication channels experience long and short duration deep fading that distorts or inhibits communication. Such prior art voice systems are only usable over a slight range of these varying channel conditions. Similar conditions can occur for image transmission, digital data and other communication systems in addition to voice coding systems.
Accordingly, it would be advantageous to have an adaptive rate communication system that maintains a continuous link despite changing channel conditions and provides a relatively low complexity and powerful channel coding operation while being simpler in design and minimizing delays through the communication system.